Method of producing a honeycomb structure

ABSTRACT

A honeycomb structure as a substrate for at least one catalyst includes a ridge arranged on at least one part of an outer surface of the honeycomb structure and extending in a direction of through-apertures thereof, and at least one notch formed in the at least one ridge. A method of producing the honeycomb structure includes the steps of extruding a ceramic batch to form a honeycomb structure having a ridge in the form of a shape extending at least one part of the honeycomb structure in cross-section, and removing at least at one part of the ridge in directions of through-apertures of the honeycomb structure to form a notch. With the arrangement of the honeycomb structure, it is possible to prevent any shifting of the honeycomb structure in directions of through-apertures of the structure and rotation of the structure about its axis relative to a converter can without closing any ends of through-apertures.

This is a division of application Ser. No. 07/384,633 filed Jul. 25,1989, now U.S. Pat. No. 5,098,763.

BACKGROUND OF THE INVENTION

This invention relates to a honeycomb structure used as a substrate forcatalysts for purifying exhaust gases from internal combustion engines,as a filter for removing fine particles in exhaust gases, and as asubstrate for various catalysts for deodorizing and/or purifying burntgases when fuels such as various gases or petroleum are burnt, and moreparticularly to a method of producing such a honeycomb structure.

FIG. 1 illustrates a catalyst converter which has been generallypractically used for purifying exhaust gases from an automobile loadedwith the converter. In order to make the converter insusceptible toviolent vibrations in use, it comprises cushion members 22-1 and 22-2and sealing members 22-3 about a honeycomb structure havingthrough-apertures 21-1 through which exhaust gases pass and platemembers 23 on upstream and downstream sides of the structure. Thecushion members and the sealing members apply forces upon the honeycombstructure in traverse or lateral directions (referred to as "radialdirections" hereinafter) of the directions of the through-apertures 21-1and the plate members 23 apply forces directly or through the cushionmembers 22-1 onto the honeycomb structure in the directions of thethrough-apertures 21-1. The honeycomb structure is fixed and heldthereat in this manner.

With such a construction of the catalyst converter, however, the cushionmembers 22-1 or the plate members 23 close some apertures 21-2 of thethrough-apertures 21-1 so that exhaust gases do not pass through theapertures 21-2, with the result that the catalyst carried by theportions of the apertures 21-2 will be inoperative.

In order to avoid this disadvantage, it has been practically proposed tohold a honeycomb structure in radial directions by seal members arrangedradially outward of the honeycomb structure for the purpose of savingcatalytic noble metals. Moreover, a honeycomb structure has been knownwhich is formed on its outer circumference with barriers adjacent atleast one end face of a sealing member as disclosed in Japanese UtilityModel Application Laid-open No. 62-179,319.

With the limitedly practically used honeycomb structure being onlyradially supported, however, high pressure is required to radiallysupport the structure in order to fix it against movement caused byviolent vibrations generated in use. It is possible to support itradially in case that thicknesses of partition walls of the ceramichoneycomb structure are comparatively thick, for example, 0.30 mm toprovide a high strength against external pressures. However, such asupporting of the honeycomb structure is not applicable to a honeycombstructure whose partition walls are relatively thin such as 0.15 mm to0.20 mm and susceptible to external pressures.

In the honeycomb structure disclosed in the Japanese Utility ModelApplication Laid-open No. 62-179,319, however, partition walls areformed separately from a main body of the honeycomb structure, so thatthere is a difficulty in adhesiveness therebetween. In more detail, evenif a thermosetting inorganic filler consists mainly of a ceramicmaterial such as alumina, silica and the like, it encounters a problemof peeling or separation in the case that a material of the honeycombstructure is cordierite, which is widely used. Such peeling results fromthermal shocks which are repeatedly generated in use because of thelarger thermal expansion of the filler than that of the honeycombstructure itself.

Therefore, although the high adhesiveness can effectively prevent themembers of the structure from being shifted in directions ofthrough-apertures of the structure without increasing the holding force,if the adhesiveness becomes less, it cannot prevent the shifting of themembers in the directions of the through-apertures.

The barriers provided on the outer circumference of the structure areeffective to prevent the shifting of the members in axial directions,but they do not serve to prevent shifting in radial directions orrotation of the members.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a honeycomb structure and amethod of producing the same which eliminate the disadvantages of theprior art to prevent shifting of the structure relative to a convertercan or vessel in directions of through-apertures of the structure aswell as radial directions or rotating directions.

In order to achieve this object, a honeycomb structure as a substratefor a catalyst according to the invention comprises a ridge arranged onat least one part of an outer surface of the honeycomb structure andextending in a direction of through-apertures thereof, and at least onenotch formed in said ridge.

In another aspect of the invention, a method of producing the honeycombstructure comprises the steps of extruding a ceramic batch to form ahoneycomb structure having a ridge in the form of a shape extending atleast at one part of the honeycomb structure in cross-section, andremoving at least one part of the ridge in directions ofthrough-apertures of the honeycomb structure to form a notch.

With the above arrangement, when the honeycomb structure having a ridgearranged on at least one part of the outer surface extending in thedirection of the through-apertures is assembled with a sealing member toform a converter, rotation of the structure about its axis relative tothe converter can is prevented by means of engagement of the sealingmember and the ridge. At the same time, shifting of the structurerelative to the converter can is also effectively prevented by stepsformed by the notch in the direction of the through-apertures.

According to the method, as the ridge having the notch is formedintegral with the honeycomb structure, it is possible to solve theproblem of adhesiveness which would occur when the ridge is formedseparately from the honeycomb structure. Moreover, as the notch can beformed by cutting, grinding or pattern-pressing, the honeycomb structureprovided with the subject features of the invention is formed in asimple manner.

The invention will be more fully understood by referring to thefollowing detailed specification and claims taken in connection with theappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are views illustrating examples of a honeycomb structureand a catalyst converter of the prior art, respectively;

FIGS. 3a-3d are perspective views illustrating various embodiments ofhoneycomb structures according to the invention, respectively;

FIGS. 4a and 4b are views illustrating steps of one embodiment of themethod of producing the honeycomb structure according to the invention;

FIG. 5 is a view showing a step of another embodiment of the methodaccording to the invention;

FIGS. 6a-6c are views illustrating steps of assembling the honeycombstructure according to the invention into a catalyst converter;

FIGS. 7a and 7b are views illustrating shapes of honeycomb structuresused in the test for comparing the invention with the prior art; and

FIGS. 8a-8c are views showing shapes of catalyst converters used in thetest for comparing the invention with the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 3a-3d illustrate in perspective views various embodiments of ahoneycomb structure according to the invention, respectively.

In the embodiment shown in FIG. 3a, a cylindrical honeycomb structure 1is partially formed on its outer circumference with a ridge 3 extendingin parallel with through-apertures 2 and having a predetermined height.The ridge 3 is partially cut to a surface in flush with the outersurface of the structure to form a notch 4.

In the embodiment shown in FIG. 3b, a honeycomb structure 1 is formedfully on its outer circumference with a plurality of ridges 3 extendingin parallel with through-apertures 2. Some of the ridges 3 are partiallycut to a surface in flush with the outer surface of the structure toform a notch 4.

In FIG. 3c, a cylindrical honeycomb structure 1 is partially formed onits outer circumference with a ridge 3 extending in parallel withthrough-apertures 2 and having a predetermined height. A part of theridge 3 is cut to form a notch 4 consisting of a plurality of groovesadjacent each other.

In the embodiment shown in FIG. 3d, edges of a ridge 3 formed on ahoneycomb structure 1 are smoothly connected with an outercircumferential surface of the structure 1. In contrast herewith, in theembodiment of FIG. 3a, the ridge 3 forms steps or shoulders relative tothe outer circumference of the structure 1.

The honeycomb structures as shown in FIGS. 3a, 3b and 3d are preferablymanufactured according to steps shown in FIGS. 4a and 4b. In otherwords, as shown in FIGS. 4a and 4b, a ceramic batch is extruded througha die 5 having a retaining cover 6 formed at its opening with a notch orformed in a predetermined shape to form an integral honeycomb structurehaving a ridge 3. Thereafter, a cutting tool 7 vertically movablyprovided relative to the honeycomb structure 1 is lowered to a positionwhere the cutting tool 7 is able to form a predetermined notch 4. Thenotch 4 is formed in this manner.

The cutting tool may be rotatable. A depth of the notch 4 can beadjusted at will by changing the position where the cutting tool islowered. In this case, by changing the shape of the cutting tool 7 thenotch can be formed such that its bottom is quite coincident with anouter circumferential surface of the honeycomb structure 1.

In the embodiment shown in FIGS. 4a and 4b, the notch 4 is formed bymeans of the cutting tool 7 immediately after the extrusion of thehoneycomb structure 1. In such a case, the notch 4 can be formed by athin wire such as piano wire without the cutting tool 7 because theextruded structure is still sufficiently soft. As an alternative, afterdrying or firing the structure, it may be cut by the cutting tool 7 orground by a grinding wheel such as a diamond wheel.

In producing the honeycomb structure 1 as shown in FIG. 3c, afterforming a honeycomb structure 1 having a predetermined ridge 3 andbefore drying it, a pattern tool 8 having serrations is pressed againstthe ridge 3 at a predetermined position to form grooves by serrations ofthe pattern tool 8 as shown in FIG. 5. After drying or firing thestructure, the notch may of course be formed by cutting or grinding.

In assembling the honeycomb structures as above descried into convertersas shown in FIGS. 6a, 6b and 6c, a honeycomb structure is encircled by asealing material or member 9 and provided with stainless wires 12 ifrequired. Thereafter the structure is incorporated in converter cans 10and 11. With such an arrangement, as shown in FIGS. 6a and 6b anyrotation of the honeycomb structure 1 about its axis is prevented byabutment of the ridge 3 against edges 9a of the sealing or holdingmaterial or member 9 and a projection 10a extending inwardly of the can10. Moreover, the shifting of the structure 1 in directions in parallelwith the through-apertures 2 is prevented by abutment of the notch 4 ofthe ridge 3 against an end 9b of the sealing member 9 and projections10b extending inwardly of the cans 10 and 11. In this embodiment, thesealing member 9 is fitted in the notch 4 of the ridge 3. In theembodiment in FIGS. 6a and 6b, a space is formed by the honeycombstructure 1 and the cans 10 and 11 spaced by the ridge 3 and projections10a and 10b.

FIG. 6c illustrates another embodiment in which a can 10 is similar inshape to the honeycomb structure 1 but slightly larger than thestructure 1 and diametrically divided into two halves and a sealingmember 9 is filled between the honeycomb structure 1 and the can 10. Thesealing member 9 is formed with slits 9c correspondingly to the notch 4of the honeycomb structure 1 so that shifting of the honeycomb structure1 in directions parallel to the through-apertures 2 is prevented by thenotch 4 and the part o the sealing member 9 between the slits 9c andfitted in the notch 4.

In this manner, the honeycomb structure 1 can be held only by thesealing member 9 without using holding means such as retainers whichtend to close the through-apertures, thereby saving cost formanufacturing the cans 10 and 11.

Moreover, if end faces of the ridge 3 and through-aperturescorresponding to the notch 4 are closed by a ceramic material or thelike, a catalytic precious metal can also be saved.

Actual examples of the honeycomb structure according to the inventionwill be explained hereinafter.

EXAMPLE 1

A cordierite batch material was extruded and fired to obtain honeycombstructures each having a 100 mm diameter, of 46.5/cm². Thicknesses ofpartition walls were 0.20 mm except those of Comparative example 2 whichwere 0.23 mm. Thicknesses of outer walls were about 0.3 mm.

A honeycomb structure of Embodiment 1 according to the invention had 48ridges 3 circumferentially arranged like petals, each having a 1 mmheight and 2 mm radii of curvatures at a top and a bottom as shown inFIG. 3b. Moreover, a notch was at a location 10 mm spaced from one endsurface of the honeycomb structure and had a 25 mm length, a 1 mm depthand a width over three ridges 3. A thickness of the outer wall isuniformly 0.3 mm which is different from the embodiment shown in FIG. 3bwhose outer wall is varied to form the ridges.

A honeycomb structure of Embodiment 2 according to the invention had oneridge 3 having a 1 mm height and a 20 mm width (circumferential length)and a notch 4 at a location 10 mm spaced from one end surface of thestructure and having a 25 mm length and a 1 mm depth as shown in FIG.3a.

A honeycomb structure of Embodiment 3 according to the invention wassimilar to that of the Embodiment 2 with exception of a 0.5 mm height ofa ridge and a 0.5 mm depth of a notch.

A honeycomb structure of Embodiment 4 according to the invention had aridge formed by extending an outer wall 0.3 mm radially outwardly over a20 mm width (circumferential length) and a notch located spaced 10 mmfrom one end surface and having a 25 mm length and a 0.3 mm depth.

A honeycomb structure of Embodiment 5 according to the invention had anaddition to the notch of the Embodiment 4 a further notch spaced 10 mmfrom the other end surface and having a 25 mm length and a 0.3 mm depth.

A honeycomb structure of Embodiment 6 according to the invention had tworidges circumferentially 45° spaced each having a 0.3 mm height and a 20mm width and each formed at its center with a notch having a length of50 mm.

On the other hand, honeycomb structures of Comparative Examples wereprepared which did not have ridges 3 and notches 4 as shown in FIG. 7b.

These honeycomb structures of Embodiments 1-6 and Comparative Examples 1and 2 were tested on various performances. The results are shown inTable 1.

                                      TABLE 1                                     __________________________________________________________________________                                   Strength against                                                                       Canning test                                   Ridge      Notch      external pressure                                                                      X Damaged                             Kind     Height mm                                                                           Number                                                                             Length mm                                                                           Number                                                                             kg/cm.sup.2                                                                            ◯ Sound                   __________________________________________________________________________    Embodiment 1                                                                           1.0   1    25    1    29˜34                                                                            ◯                         Embodiment 2                                                                           1.0   1    25    1    18˜25                                                                            X                                     Embodiment 3                                                                           0.5   1    25    1    23˜26                                                                            X                                     Embodiment 4                                                                           0.3   1    25    1    15˜22                                                                            X                                     Embodiment 5                                                                           0.3   1    25    2    22˜26                                                                            X                                     Embodiment 6                                                                           0.3   2    50    1 × 2                                                                        18˜24                                                                            X                                     Compar-                                                                             -1 --    --   --    --   20˜31                                                                            X                                     ative -2                                                                      example 1                                                                     Compar-                                                                             -1 --    --   --    --   78˜96                                                                            ◯                         ative -2                                                                      example 2                                                                     __________________________________________________________________________                 Heated vibration test X Structure shifted ◯ Not                   shifted                                                                                    Rotat-            Rotat-                                         Holding                                                                            Clear-                                                                            Axial                                                                             ing Holding                                                                            Clear-                                                                            Axial                                                                             ing                                             member                                                                             ance*                                                                             shift-                                                                            shift-                                                                            member                                                                             ance*                                                                             shift-                                                                            shift-                             Kind         (FIG.)                                                                             mm  ing ing (FIG.)                                                                             mm  ing ing                                __________________________________________________________________________    Embodiment 1 6a   3.5 ◯                                                                     ◯                                                                     --   --  --  --                                 Embodiment 2 6a   3.5 ◯                                                                     X   6b   3.5 ◯                                                                     ◯                      Embodiment 3 6a   3.5 ◯                                                                     X   6b   3.5 ◯                                                                     ◯                      Embodiment 4 6a   3.5 X   X   6b   3.5 X   X                                  Embodiment 5 6a   3.5 X   X   6b   3.5 X   X                                  Embodiment 6 6a   3.5 X   X   6b   3.5 ◯                                                                     ◯                      Compar-   -1 6a   3.5 X   X   6c   3.5 X   X                                  ative     -2                  8c   3.5 ◯                                                                     ◯                      example 1                                                                     Compar-   -1 6a   2.2 X   X   6c   3.5 X   X                                  ative     -2          ◯                                                                     ◯                                                                     6c   2.2 ◯                                                                     ◯                      example 2                                                                     __________________________________________________________________________     Note                                                                          *Clearance between can and structure                                     

In an external pressure strength test, aluminum plates having athickness of about 20 mm were applied through urethane sheets having athickness of about 0.5 mm to end surfaces of a honeycomb structure and acircumferential surface of the structure was hermetically surrounded byan urethane tube having a wall thickness of about 0.5 mm. The honeycombstructure was accommodated in a vessel filled with water and thepressure in the vessel was raised slowly. The pressure in the vesselwhen a sound was generated due to damage of the honeycomb structure wasmeasured.

In a canning test, a ceramic mat as a holding member shown in FIG. 6bwas wound about a honeycomb structure and this assembly was inserted ina jig. The jig had an inlet whose inner diameter was larger than that ofan outlet to form a tapered jig. The inner diameter of the outlet of thejig was substantially the same as an inner diameter of an inlet of asteel pipe or can. The outlet of the jig was attached to the inlet ofthe can and the honeycomb structure was forced into the can by means ofa hydraulic ram. After the honeycomb structure was removed out of thecan, an external appearance of the structure was observed. The ceramicmat had a thickness of 4.9 mm. The inner diameter of the steel pipe orcan was 104.4 mm.

In a heated vibration test, a holding member shown in Table 1 was woundabout a honeycomb structure and the assembly was forced into a steelpipe or can having an inner diameter forming a clearance between thehoneycomb structure and a can with a flange welded thereto. Thereafter,hollow conical members were bolted to both ends of the can and tested.

A heated gas at 800° C. by means of a propane burner was caused to flowagainst the can for two minutes so as to heat the honeycomb structure.Then air at room temperature was caused to flow against the can for twominutes so as to cool the honeycomb structure. One cycle formed by sucha heating and a cooling was repeated to 50 cycles while the can wassubjected to vibrations of 0-20 g with 200 Hz. Thereafter, the hollowconical members were removed from the can from which the honeycombstructure was removed to observe an external appearance of thestructure.

From the results shown in Table 1, it is evident that the honeycombstructures resisting the heated vibration test simulated as actuallyused conditions are (1) those supported in directions in parallel withthrough-apertures as well in a hitherto used manner, (2) those includingnarrow clearances between the honeycomb structures and can such as 2.2mm or supported by high holding pressure, and (3) those of theembodiments according to the invention having ridges whose heights aremore than 0.5 mm. The honeycomb structures according to the inventioncan be held only on their circumferential surfaces even by low holdingpressures. Even the structures having the ridges whose heights are 0.3mm according to the invention can resist the heated vibration testdepending upon conditions to be subjected. The effectiveness of theridges having the notches according to the invention has beenascertained. It has been moreover found that catalytic noble metal canbe saved approximately 8% in comparison of effective volumes ofcatalysts between those of Embodiments according to the invention andComparative examples of the prior art.

It is to be understood that the invention is not limited to the aboveembodiments and various changes and modifications may be made in theinvention without departing from the scope of the invention. Forexample, sizes of the ridges and the notches are not limited to thoseillustrated and explained because they are of design choice to bedetermined by used conditions and widths in through-aperture directionsof cushion members abutting against the ridges and notches.

Although the cross-sectional shape of the ceramic honeycomb structuresis correctly circular in the above embodiments, it is not limited tocircular and may be for example elliptical. Moreover, the sectionalshape of cells of the honeycomb structures are not limited to square asin the embodiments and the material thereof is not limited to cordieriteused in the above embodiments. Furthermore, both ends ofthrough-apertures of the honeycomb structures may of course bealternately closed.

As can be seen from the above description, the honeycomb structureaccording to the invention comprises at least one ridge having at leastone notch on its outer circumferential surface to prevent shifting inthrough-aperture directions and rotation about its axis, therebyenhancing the reliability of a catalyst converter incorporating thehoneycomb structure therein. Moreover, the honeycomb structure accordingto the invention can be incorporated in a converter without closing anyends of through-apertures so that all the catalytic noble metal carriedby the honeycomb structure is effectively used for the purpose.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details can be made therein without departing from the spirit andscope of the invention.

What is claimed is:
 1. A method of producing a honeycomb structurecomprising the steps of:extruding a ceramic batch to form a honeycombstructure having at least one ridge arranged on at least one part of anouter surface of said honeycomb structure and extending in a directionof through-apertures of said honeycomb structure; and removing at leastone part of said ridge to form a notch substantially perpendicular tosaid direction of through-apertures.
 2. The method of claim 1, whereinsaid notch is formed by at least one operation selected from the groupconsisting of cutting and grinding.
 3. The method of claim 1, whereinsaid notch consists of a plurality of grooves formed by pressing apattern tool having serrations against the ridge of the honeycombstructure while the honeycomb structure is in a green state.
 4. Themethod of claim 1, further comprising the step of drying said honeycombstructure and said notch is formed after said drying step.
 5. The methodof claim 1, further comprising the step of firing said honeycombstructure and said notch is formed after said firing step.